Re: [PATCH v2] mm: fix swap offset when replacing shmem page

[Hugh Dickins <hughd@google.com>]

On Sun, 18 Nov 2018, Yu Zhao wrote:

> We used to have a single swap address space with swp_entry_t.val
> as its radix tree index. This is not the case anymore. Now Each
> swp_type() has its own address space and should use swp_offset()
> as radix tree index.
> 
> Signed-off-by: Yu Zhao <yuzhao@google.com>

This fix is a great find, thank you! But completely mis-described!
And could you do a smaller patch, keeping swap_index, that can go to
stable without getting into trouble with the recent xarrifications?

Fixes: bde05d1ccd51 ("shmem: replace page if mapping excludes its zone")
Cc: stable@vger.kernel.org # 3.5+

Seems shmem_replace_page() has been wrong since the day I wrote it:
good enough to work on swap "type" 0, which is all most people ever use
(especially those few who need shmem_replace_page() at all), but broken
once there are any non-0 swp_type bits set in the higher order bits.

> ---
>  mm/shmem.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d44991ea5ed4..685faa3e0191 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1509,11 +1509,13 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
>  {
>  	struct page *oldpage, *newpage;
>  	struct address_space *swap_mapping;
> -	pgoff_t swap_index;
> +	swp_entry_t entry;

Please keep swap_index as well as adding entry.

>  	int error;
>  
> +	VM_BUG_ON(!PageSwapCache(*pagep));
> +

I'd prefer you to drop that, it has no bearing on this patch;
we used to have it, along with lots of other VM_BUG_ONs in here,
but they outlived their usefulness, and don't need reintroducing -
they didn't help at all to prevent the actual bug you've found.

>  	oldpage = *pagep;
> -	swap_index = page_private(oldpage);
> +	entry.val = page_private(oldpage);

entry.val = page_private(oldpage);
swap_index = swp_offset(entry);

>  	swap_mapping = page_mapping(oldpage);
>  
>  	/*
> @@ -1532,7 +1534,7 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
>  	__SetPageLocked(newpage);
>  	__SetPageSwapBacked(newpage);
>  	SetPageUptodate(newpage);
> -	set_page_private(newpage, swap_index);
> +	set_page_private(newpage, entry.val);

Yes.

>  	SetPageSwapCache(newpage);
>  
>  	/*
> @@ -1540,7 +1542,8 @@ static int shmem_replace_page(struct page **pagep, gfp_t gfp,
>  	 * a nice clean interface for us to replace oldpage by newpage there.
>  	 */
>  	xa_lock_irq(&swap_mapping->i_pages);
> -	error = shmem_replace_entry(swap_mapping, swap_index, oldpage, newpage);
> +	error = shmem_replace_entry(swap_mapping, swp_offset(entry),
> +				    oldpage, newpage);

I'd prefer to omit that hunk, to avoid the xa_lock_irq() in the context;
the patch is just as good if we keep the swap_index variable.

>  	if (!error) {
>  		__inc_node_page_state(newpage, NR_FILE_PAGES);
>  		__dec_node_page_state(oldpage, NR_FILE_PAGES);
> -- 
> 2.19.1.1215.g8438c0b245-goog

Thanks,
Hugh